In view of numerous factors such as higher energy prices and environmental concerns, the production of value-added products (such as pipeline-quality substitute natural gas, hydrogen, methanol, higher hydrocarbons, ammonia and electrical power) from lower-fuel-value carbonaceous feedstocks (such as petroleum coke, resids, asphaltenes, coal and biomass) is receiving renewed attention.
Such lower-fuel-value carbonaceous feedstocks can be gasified at elevated temperatures and pressures to produce a synthesis gas stream that can subsequently be converted to such value-added products.
“Conventional” gasification processes, such as those based on partial combustion/oxidation and/or steam gasification of a carbon source at elevated temperatures and pressures (thermal gasification), generate syngas (carbon monoxide+hydrogen, lower BTU synthesis gas stream) as the primary product (little or no methane is directly produced). The syngas can be directly combusted for heat energy, and/or can be further processed to produce methane (via catalytic methanation, see reaction (III) below), hydrogen (via water-gas shift, see reaction (II) below) and/or any number of other higher hydrocarbon products.
One advantageous gasification process is hydromethanation, in which the carbonaceous feedstock is converted in a fluidized-bed hydromethanation reactor in the presence of a catalyst source, syngas (carbon monoxide and hydrogen) and steam at moderately-elevated temperatures and pressures to directly produce a methane-enriched synthesis gas stream (medium BTU synthesis gas stream) raw product, which can then also be directly combusted, further processed to enrich the methane content, used to produce hydrogen and/or used to produce any number of other hydrocarbon products.
Such lower-fuel-value carbonaceous feedstocks can alternatively be directly combusted for their heat value, typically for generating steam and electrical energy (directly or indirectly via generated steam).
In the above uses, the raw particulate feedstocks are typically processed by at least grinding to a specified particle size profile (including upper and lower end as well as dp(50) of a particle size distribution) suitable for the particular fluidized-bed or other gasification operation. Typically particle size profiles will depend on the type of bed, fluidization conditions (in the case of fluidized beds, such as fluidizing medium and velocity) and other conditions such as feedstock composition and reactivity, feedstock physical properties (such as density and surface area), reactor pressure and temperature, reactor configuration (such as geometry and internals), and a variety of other factors generally recognized by those of ordinary skill in the relevant art.
“Low-rank” coals are typically softer, friable materials with a dull, earthy appearance. They are characterized by relatively higher moisture levels and relatively lower carbon content, and therefore a lower energy content. Examples of low-rank coals include peat, lignite and sub-bituminous coals. Examples of “high-rank” coals include bituminous and anthracite coals.
In addition to their relatively low heating values, the use of low-ranks coals has other drawbacks. For example, the friability of such coals can lead to high fines losses in the feedstock preparation (grinding and other processing) and in the gasification/combustion of such coals. Such fines must be managed or even disposed of, which usually means an economic and efficiency disadvantage (economic and processing disincentive) to the use of such coals. For very highly friable coals such as lignite, such fines losses can approach or even exceed 50 wt % of the original material. In other words, the processing and use of low-rank coals can result in a loss (or less desired use) of a material percentage of the carbon content in the low-rank coal as mined.
It would, therefore, be desirable to find a way to efficiently process low-rank coals to reduce fines losses in both the feedstock processing and ultimate conversion of such low-rank coal materials in various gasification and combustion processes.
Low-rank coals that contain significant amounts of impurities, such as sodium and chlorine (e.g., NaCl), may actually be unusable in gasification/combustion processes due to the highly corrosive and fouling nature of such components, thus requiring pretreatment to remove such impurities. Typically the addition of such a pretreatment renders the use of sodium and/or chlorine contaminated low-rank coals economically unfeasible.
It would, therefore, be desirable to find a way to more efficiently pretreat these contaminated low-rank coals to removed a substantial portion of at least the inorganic sodium and/or chlorine content.
Low-rank coals may also have elevated ash levels, and thus lower useable carbon content per unit raw feedstock. In addition, elevated silica/alumina levels can bind and interfere with many alkali-metal catalysts used in hydromethanation processes, requiring more stringent (and more highly inefficient) and increased amounts of catalyst recovery and catalyst makeup.
It would, therefore, be desirable to find a way to more efficiently pretreat these low-rank coals to reduce overall ash content and, to the extent possible, reduce the alumina component of ash content.
Also, low-ranks coals tend to have lower bulk density and more variability in individual particle density than high-rank coals, which can create challenges for designing and operating gasification and combustion processes.
It would, therefore, be desirable to find a way to increase both particle density and particle density consistency of low-rank coals, to ultimately improve the operability of processes that utilize such low-rank coals.